1. Field of the Invention
The present invention relates to a dark field inspection system and in particular to forming a ring illumination for this inspection system.
2. Related Art
The surface on a semiconductor wafer is optimally flat. However, even for a blank wafer, some residual roughness is inevitably present. This roughness, which may be only 2 nm or less (i.e. much less than the wavelength of light used for inspection), can still result in undesirable fluctuations in detected scattered light at an imaging sensor in a dark field inspection system. These fluctuations can be characterized as a noise floor and are referenced herein as “speckle”. For wafer inspection, the speckle is effectively the limiting factor of sensitivity of the imaging sensor. That is, small particles (e.g. defects) that might otherwise be detected may be obfuscated by speckle.
Conventional methods of dark field wafer inspection have not been devised to overcome speckle, including edge contrast (EC) modes of broadband systems or laser dark field inspection systems. Unfortunately, an EC mode of a broadband system uses a low brightness broadband light source, which results in a lower illumination level at the imaging sensor. Moreover, an EC mode of the broadband system has an inherently limited numerical aperture (NA) available for defect detection because the NA is used for both illumination and imaging. This limited NA can result in low optical resolution, and relatively low collection efficiency for the scattered light.
A typical laser dark field inspection system, such as the Puma family of products offered by KLA-Tencor, uses oblique light incidence, which creates a relatively large line width (e.g. on the order of 1 um) which can somewhat limit the resolution. Moreover, a typical laser dark field inspection system uses a single angle of illumination, which results in strong spatial coherence. Strong spatial coherence can result in a relatively large level of roughness induced fluctuations (or speckle), which can affect the ultimate sensitivity of the system to real defects.
Therefore, a need arises for a dark field inspection system that can significantly improve defect detection sensitivity.